1. Field of the Invention
The present invention relates to an active load circuit for a voltage gain amplifier, and more particularly, for a high-frequency low-voltage complementary-metal-oxide-semiconductor (CMOS) voltage gain amplifier with a wide bandwidth and a high gain characteristic.
2. Description of the Related Art
When a circuit for a CMOS voltage gain amplifier is designed, various factors, such as a voltage gain, operating frequency bandwidth, low distortion, an input signal range for obtaining a desired linearity, a noise characteristic, and a signal to noise ratio (SNR) should be considered.
To achieve a high gain, a voltage gain amplifier even in low voltage operation must have its high output impedance with an output load element. A resistor is generally used as the output load element. FIG. 1 is a circuit diagram of a conventional voltage gain amplifier using resistors as its output load elements.
Referring to FIG. 1, a voltage gain amplifier (10) comprises a first MOS transistor (15) and a second MOS transistor (20), which are connected to each other in a differential pair configuration.
A first input voltage +Vid is input signal to the gate of the first MOS transistor 15, and a second input voltage −Vid, which is a differential signal of the first input voltage +Vid, is input signal to the gate of the second MOS transistor 20. The source of the first MOS transistor 15 is connected to the source of the second MOS transistor 20, and a current source for supplying a predetermined bias is connected to the sources of the first and second MOS transistors 15 and 20. The drain of the first MOS transistor 15 and the drain of the second MOS transistor 20 are connected to load elements connected to a power supply voltage VDD, for example, resistors 25a and 25b, respectively.
A common mode voltage signal Vic is a voltage source that provides DC voltage levels of the first and second input voltages +Vid and −Vid.
In this differential voltage gain amplifier 10, if signals of the first and second input voltages +Vid and −Vid are inputted to the first and second MOS transistors 15 and 20, respectively, the first and second MOS transistors 15 and 20 amplify a difference between the first and second input voltages +Vid and −Vid and output the amplified signal via output terminals Vo.
However, the values of the output resistors 25a and 25b are limited so that a desired voltage gain cannot be achieved even in a low voltage operation. Also, the voltage gain amplifier 10 exhibits its bandwidth characteristic in that a voltage gain remains constant in a certain frequency range, but it is dramatically reduced in a frequency range higher than the certain frequency range. Thus, the voltage gain amplifier cannot obtain a sufficiently wide bandwidth.
Further, the voltage gain amplifier mentioned is greatly dependent on the temperature variations or power supply voltage variations. In particular, a CMOS voltage gain amplifier considering the threshold voltage Vth of a MOS transistor, it is difficult to have a desired input/output signal range due to comparatively high its value of the resistors 25a and 25b in a low-voltage operation. For these reasons, the conventional voltage gain amplifier scheme with the resistor loads cannot achieve both a low-power characteristic and low-voltage/high-frequency operation.